426 Climate of the Glacial Period. (October, 
now, the isotherm of 50° F., passing across the south of 
England near the latitude of London and the Bristol 
Channel, sweeps south-westwardly across the Atlantic, and 
reaches to about Baltimore, in North America, so in the 
Glacial period the margin of the ice, flowing southwards, 
attained nearly the same limits; indicating that the warm 
waters from the tropics then, as now, were deflected against 
the western coasts of Europe by the rotation of the earth, 
and gave them a higher temperature than the same lati- 
tudes on the eastern coasts of America. The sea teems 
with life, and it is not possible that this current could have 
flowed over any part of Europe without leaving many me- 
morials of its course behind it. But even if it had been 
diverted, and a cold current brought icebergs from the Arctic 
regions past the British Isles, how could that, or any modi- 
fication of such a theory, cause continental ice to reach the 
sea-level in lat. 39° in North America? I cannot imagine 
any alteration of the present coast-lines that could cause a 
greater curve in the isothermal lines than at present exists 
in the North Atlantic ; and to assume that during the Glacial 
period the warm and cold currents shifted their position all 
round the hemisphere, so as to bring every part, at one time 
or other, within a greater extreme of cold than now any- 
where prevails, is to call for an amount of movement in 
the earth’s crust that no evidence warrants nor analogy 
suggests. 
Whilst Lyell, in his latest works,* adheres to his opinion 
that former changes of climate have been chiefly governed 
by geographical conditions, he candidly admits that since he 
first attempted to solve the problem, our knowledge of the 
subject has vastly increased, and that it has assumed a some- 
what new aspect, so that he now considers it probable that 
astronomical causes may have combined with geographical 
changes to produce an exaggeration of cold in both hemi- 
spheres. The principal of these astronomical theories I 
shall now take into consideration, but I shall have in the 
sequel—when I come to show what bearing the fa¢ts of the 
Early Tertiary period have on the discussion—to make some 
further remarks upon the insufficiency of geographical 
changes to account for the great oscillations of temperature 
of which we have geological proofs. 
2. Theory of an Increase of the Ellipticity of the Earth’s 
Orbit.—Mr. Croll, in a series of papers published in the 
‘** Philosophical Magazine,” has advocated, with great ability 
* Principles of Geology, 1872, pp. 173 and 284. 
